Extreme scale and indexes are not at all a happy couple.

Doing away with database-managed indexes at scale makes for simpler queries and predictable latencies now and in the future. These indexes have costs associated with them which become detrimental as scale rises.

Application-managed indexes along with the required codebase discipline results in reduced bottlenecks, minimal latencies, and better parallelism. One can even parallelize the index-fetch phase itself, not to mention all the other goodies that come with it. This results in a different trade-off as cost moves to the application where it is controlled and therefore leveraged.

Note: I am talking generic database architecture and not just about Postgres.